(1) Technical Field
The present invention relates to an internally heated cathode. More specifically, the internally heated cathode that comprises an a cavity structure, where at least a portion of the cavity structure forms an emission material portion, the cavity structure defining a cavity, and a heater disposed within the cavity, providing for an efficient, durable, and long lasting cathode that requires less power.
(2) Description of Related Art
Cathodes have a variety of applications in the art, including, but not limited to: generating in-space plasma; neutralizing plumes; functioning as the cathode for noble gas, hydrogen, and other discharges. Additionally, cathodes function as the electron source for a variety of equipment that employ an ion beam, such as ion beam milling systems, scanning electron microscope (SEMs), transmission electron microscopes (TEMs), electron lithography systems, electron accelerators, x-ray sources, free electron lasers, accelerators, melting furnaces, and other standard and custom equipment. Cathode materials are generally heated to temperatures greater than 1000 degrees Celsius to produce sufficient light or electron emissions for a large range of applications. Many existing cathode assemblies operate at low efficiencies due to the high energies they require to heat the cathodes to sufficiently high temperatures. For many cathode applications, it is desirable that the cathode assembly is exposed directly to the environment, however, many existing cathode assemblies are prone to self-contamination, which destroys or severely reduces the cathode's emission portionabilities.
A typical cathode used in the art is the Vogel-mounted design 100, illustrated in FIG. 1, which supports an emitter material 102 with two curved support arms 104 rising from a base 106, each arm 104 supporting a pyrolytic carbon block heater 108 on the upper end such that the heaters 108 are in direct contact with the emitter material 102.
The Vogel-mounted design is advantageous in that the pyrolytic carbon block heater 108 provides for high resistance because of its perpendicular orientation with respect to direction of the electrical current. Additionally, the direct contact of the heater 108 to the emission material 102 allows for direct conduction of heat to the emission material, which results in greater thermal efficiency. Finally, the block heater design is more durable than a filament heater used in most cathode emitters. To provide a resistance with a filament heater similar to the level of the block heater, a filament would have to be extremely thin and fragile. Higher resistance cathode heaters are attractive since they reduce the current required to generate a given amount of heat power.
However, the Vogel-mounted design has several distinct disadvantages. Foremost is the loss of heat radiated into the environment from the exposed surfaces of the heater 108. The loss of heat decreases the efficiency of the cathode and consequently requires a higher power to achieve a desired level of emission. This heat loss also makes the Vogel-mounted design undesirable in situations where a large amount of heat emission cannot be tolerated. Additionally, the Vogel-mounted design is prone to self-contaminating due to evaporation and ion-bombardment of the base 106. With typical base materials such as ceramics and MACOR® (a high-temperature glass ceramic insulator made by Corning, Inc. of Corning, N.Y.), contaminants evaporate and sputter from the base 106 as it is eroded by ion-bombardment and contaminate the emitter surface 110 of the emission material 102. Contamination of the emitter surface results in a decrease in cathode efficiency or a complete loss of cathode emission. Another limitation of the Vogel-mounted design is its structural stability. Since the device is simply held in place by two freestanding, curved arms 104, the cathode is susceptible to slight external movement, vibration, and shock. The cathode assembly becomes increasingly more fragile with increased operating time.
Another common cathode design is the conventional aft heater design 200, illustrated in FIG. 2, which uses a filament-type heater 204 behind, but not in contact with, the emission material 202. The emission material 202 is supported by an aft assembly 206 that encloses the heater 204.
The aft-heater design is advantageous in that the design provides some mitigation of self-poisoning, as the heater 204 is essentially enclosed within the aft assembly 206 and potential contaminants, such as MACOR®, are not directly exposed to the emitter environment. Additionally, the design is more stable in that the structure does not rely on the horizontal pressure provided by two support arms (as with the Vogel-mounted cathode—see FIG. 1), but instead relies on a vertically-stacked design with one component supporting another.
The aft-mounted heater cathode has several disadvantages. As with the Vogel-mounted design, the entire cathode 200 is exposed directly to the environment, creating similar inefficiencies with regard to required energy and superfluous heat radiation. Only the portion of the heater 204 surrounded by the aft assembly 206 prevents heat from radiating to the environment. Additionally, the heater 204 is a filament heater, which requires high current to achieve a desired heat in comparison with the block heater in the Vogel-mounted cathode. Furthermore, as the heater 204 is not in direct contact with the emission material 202, the thermal efficiency of the design is much less than that of the Vogel-mounted design.
As with the aforementioned designs, most cathodes require a high current to overcome the inefficiencies described above. Heat radiating into the environment from exposed heaters and the lack of direct contact between a heater and emission material are the primary sources of cathode inefficiency. Consequently, many cathode designs cannot be used with applications that require low power and low heat loss. Additionally, the potential for self-contamination of the emitting surface of the emission material in certain environments, such as plasma discharges, also significantly limits the lifespan, efficiency, and usefulness of many cathode technologies. Finally, the fragile nature of many cathodes with thin heaters and weak support structures does not provide for a cathode that can survive the environmental stresses of applications outside the laboratory, such as the mechanical shocks and vibrations experienced by space equipment during launch, or by a portable cathode emitter device during transport.
Therefore, a need exists in the art for a cathode that minimizes heat loss with a thermally efficient design so as to require minimal energy and provide maximum efficiency. Additionally, a need exists for a cathode that is not prone to self-contamination to assure an extended lifespan and high efficiency throughout the life of the cathode. Finally, a need exists for a durable cathode with these properties that is designed to withstand environmental stresses and the harsh conditions of cathode operation.